Digital circuit

ABSTRACT

A precharge circuit which suppresses a peak of a charge current in conducting a precharge operation is disclosed. 
     The precharge circuit comprises a precharge transistor for feeding the precharge current and means for generating a precharge control signal which changes slowly only when the precharge control signal is near an intermediate level of the specified binary levels of the precharge signal and changes quickly when the precharge control signal is not near the intermediate level.

BACKGROUND OF THE INVENTION

The present invention relates to a digital circuit, and more particularly to a precharge circuit for a digital circuit employing field-effect transistors.

Digital circuits widely adopt the precharge technique in which predetermined circuit nodes are all charged to a predetermined potential. Subsequently, charges accumulated at the selected circuit nodes are discharged to determine logic levels. This precharge technique is advantageous in that there is low power consumption and high speed operation.

For example, in a memory circuit, a plurality of digit lines to which memory cells are connected, are precharged from a precharge voltage source through precharge transistors coupled between the respective digit lines and the precharge voltage source prior to each operation. In performing the precharge, a precharge control signal is commonly applied to the precharge transistors. The precharge control signal is usually generated as an output signal of an inverter which receives a basic control signal such as a chip enable signal and with a predetermined slope it changes from the one of the binary logic levels which makes the precharge transistors non-conducting to the other of the binary logic levels which makes the precharge transistors conducting.

When the precharge control signal changes from the above one of the binary logic levels towards the other of the binary logic levels, a potential at the gates of the precharge transistors similarly changes and the precharge transistors become conducting to feed circuit nodes such as digit lines with currents. The potential at the circuit nodes to be charged is gradually raised as time elapses. This raise of potential at the circuit nodes functions to reduce the gate-source voltage bias of the precharge transistors. Therefore, amount of currents fed through the precharge transistors is increased in accordance with the shift in potential of the precharge control signal towards the other of the binary logic levels until a predetermined time point. It is then decreased in accordance with the potential raise at the circuit nodes to be charged. Accordingly, the amount of current fed throughout the precharge inevitably takes a peak value. In general, if a current having a large peak value flows in a circuit, then various harmful noises are generated and in the worst case malfunction of the circuit would be caused.

In order to reduce the peak value of the current, conductances of the precharge transistors could be reduced. Also, the rate at which the precharge voltage is applied can be reduced to moderate the peak value of the precharge current. However, if this technique were employed, there would be a disadvantage in that the time required for completing the precharge would be prolonged, resulting in a low speed operation.

It is one object of the present invention to provide a precharge circuit in which the peak value of a charging current can be made small without prolonging the charging time.

SUMMARY OF THE INVENTION

In the present invention, as a precharge control signal is applied to a precharge transistor for controlling the feed of a precharge current to a circuit portion to be charged, such a signal when in the process of its level change from the one of the binary logic levels which makes the precharge transistor non-conducting to the other of the binary logic levels which makes the transistor conducting, is temporarily held at an intermediate level of the binary logic levels, or its change rate towards the other level is temporarily made small near the intermediate level.

In other words, the precharge control signal employed in the present invention is once changed from the one of the binary logic levels to the intermediate level which makes the precharge transistor slightly conducting at a relatively high speed and it is temporarily held at the intermediate level for a predetermined period or it is slowly changed near the intermediate level at a low speed, and then it is changed from the intermediate level to the other of the binary logic levels at a relatively high speed.

According to the present invention, since the precharge control signal is once held at the intermediate level or its change rate is made very small near the intermediate level, effective conductance of the precharge transistor at that time is limited to a relatively low value or moderated and hence the peak value of the current fed through the precharge transistor is also limited to a small value. As a result, the wave form of the precharge current is made moderate throughout the precharge operation without prolonging the period of time of the precharge operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a precharge circuit in the prior art;

FIG. 2 is a timing chart for explaining the operation of the circuit of FIG. 1;

FIG. 3 is a circuit diagram of a first preferred embodiment of the present invention;

FIG. 4 is a timing chart for explaining the operation of the circuit of FIG. 3;

FIG. 5 is a circuit diagram of one example of an application of the present invention;

FIG. 6 is a timing chart for explaining the operation of the circuit of FIG. 5;

FIG. 7 is a circuit diagram of a second preferred embodiment of the present invention;

FIG. 8 is a circuit diagram of a third preferred embodiment of the present invention;

FIG. 9 is a timing chart for FIG. 8; and

FIG. 10 is a timing chart for representing the comparison between the prior art and the present invention.

Now with reference to the drawings, the present invention will be explained in detail. In the following explanation, a CMOS structure in which N-channel field effect transistors and P-channel field effect transistors are included is employed by way of example and the logic "1" or high level corresponds to a power supply voltage (Vcc) while the logic "0" or low level corresponds to the ground potential.

First, with reference to FIGS. 1 and 2 a precharge circuit according to the prior art will be described.

In FIG. 1, a precharge signal generator portion 11 for generating a precharge control signal φ₃ and a portion 12 forming the precharge circuit is illustrated. In this circuit, P-channel transistors P₁₁ and P₁₂, and N-channel transistors N₁₁ and N₁₂ are employed, and a node NO11 is precharged. The precharge signal generator portion 11 is constructed of a CMOS inverter circuit consisting of the transistors P₁₁ and N₁₁.

A description will now be made of the operation of the circuit of the prior art with reference to a timing chart shown in FIG. 2. First, when a first control signal φ₁ is at "1"-level (Vcc), the transistor N₁₂ is turned ON to bring the node NO11 to be charged to "0"-level (ground level). Subsequently, when the first control signal φ₁ takes "0"-level, the transistor N₁₂ is turned OFF, and then if a second control signal φ₂ becomes "1"-level, then the precharge control signal φ₃ falls in potential gradually from "1"-level (Vcc potential) and takes "0"-level, and hence the transistor P₁₂ is turned ON to start charging. As a result, the node NO11 to be charged is brought to a charged level, i.e., "1"-level. Next, when the second control signal φ₂ becomes "0"-level, the precharge control signal φ₃ becomes "1" -level, hence the transistor P₁₂ is turned OFF. Subsequently when the first control signal φ₁ becomes "1"-level, the node NO11 to be charged is returned to "0"-level through the transistor N₁₂.

The above-mentioned circuit in the prior art had the following disadvantage. That is, in FIG. 2, when the precharge control signal φ₃ changes from "1"-level to "0"-level, the node NO11 to be charged would be raised in potential, but at this moment a large peak value would arise in the charging current I_(cc1) of the NO₁₁. In general, if a current having a large peak value flows in a circuit, then various harmful noises are generated thereby, and so, it is undesirable. In order to reduce this peak value, a method could be employed reducing the size of the transistor N₁₁ thereby moderating the slope of the precharge signal φ₃. Alternatively the size of the charging transistor P₁₂ could be reduced. However, if these methods are employed, there is a disadvantage in that the time required for charging the node NO₁₁ to be charged is prolonged.

Now description will be made on a first preferred embodiment of the present invention with reference to FIGS. 3 and 4.

As shown in FIG. 3, a precharge circuit is basically composed of a precharge signal generator portion 30 for generating a precharge control signal φ₄ and a precharge portion 35 responsive to the precharge control signal φ₄ for charging a node NO₃₇. The precharge portion 35 includes a P-channel transistor P₃₂ as a precharge transistor coupled between the node NO₃₇ and a power supply terminal V_(cc) and an N-channel transistor N₃₅ coupled between the node NO₃₇ and the ground potential. A circuit structure of the precharge portion 35 is basically the same as the portion 12 of FIG. 1. The transistor P₃₂ charges the node NO₃₇ in response to "0" or low level of the precharge control signal φ₄. The N-channel transistor N₃₅ discharges the charge at the node NO₃₇ in response to "1" or high level of a first control signal φ₁. The existense of the transistor N₃₅ is not essential to the present invention.

The precharge control signal generator portion 30 receives a second control signal φ₂ as a basic signal for controlling the precharge operation and generates the precharge control signal φ₄. The precharge control signal generator portion 30 includes a P-channel transistor P₃₁ coupled between the power supply terminal (V_(cc)) and an output node NO₃₀ and having a gate receiving the signal φ₂, N-channel transistors N₃₁ and N₃₂ connected in series between the output node NO₃₀ and the ground potential, N-channel transistors N₃₃ and N₃₄ connected in series between the output node NO₃₀ and the ground potential. Gates of the transistors N₃₁ and N₃₃ receive the signal φ₂. For controlling the transistors N₃₂ and N₃₄, delay circuits 31 and 32, a NAND gate 34 and an inverter 33 are provided. The delay circuit 31 receives the signal φ₂. An output signal of the delay circuit 31 is applied to a gate of the transistor N₃₄, a first input terminal of the NAND gate 34, and an input terminal of the delay circuit 32. An output of the delay circuit 32 is fed to a second input terminal of the NAND gate 34 through the inverter 33. An output signal of the NAND gate 34 is applied to a gate of the transistor N₃₂.

In this circuit, an effective conductance of the series circuit of the transistors N₃₁ and N₃₂ is set far larger than that of the series circuit of the transistors N₃₃ and N₃₄. For achieving this the conductances of the transistors N₃₂ and N₃₄ are made substantially the same, while the conductance of the transistor N₃₁ is made larger than that of the transistor N₃₃.

The operation of this preferred embodiment will now be explained with reference to a timing chart shown in FIG. 4.

When the first control signal φ₁ is at "1"-level, the transistor N₃₅ is turned ON and brings the node NO₃₇ to be charged to "0"-level i.e., discharged. Subsequently the first control signal φ₁ becomes "0"-level and the transistor N₃₅ is turned OFF. In this instance, the signal φ₂ is at "0" level, and the output signal at a node NO₃₄ of the delay circuit 31 is still at "0" level, and therefore the output signal at a node NO₃₂ of the gate 34 is at "1" level. Therefore, the transistor N₃₂ is conducting. Next, when the second control signal φ₂ changes from "0"-level to "1"-level, then the transistor P₃₁ is turned OFF and the transistors N₃₁ and N₃₃ are turned ON, so that the precharge control signal φ₄ begins to fall in potential from "1"-level (V_(cc) potential) with a large gradient m₃₁ that is determined by the serial ON resistances of the transistors N₃₁ and N₃₂. Thereby the transistor P₃₂ is turned ON to start charging of the node NO₃₇ to be charged. Then, as the potential of the precharge signal φ₄ falls, the conductance of the transistor P₃₂ rises, and hence, the charging current I_(cc3) for the node NO₃₇ to be charged is gradually increased. On the other hand, after the second control signal φ₂ is turned to "1"-level, the node NO₃₄ is brought to "1"-level via the delay circuit 31, and hence the transistor N₃₄ is turned ON. In this instance, the node NO₃₂ is brought to "0"-level through the NAND circuit 34 because the node NO₃₄ is at "1" level and the node NO₃₅ is still at "0" level by a delay time of the delay circuit 32 to set the node NO₃₆ at "1" level. Hence, the transistor N₃₂ is turned OFF. (The moment when the transistor 32 is turned OFF is indicated by t₃₁.) In other words, at the moment t₃₁, the current path which causes potential drop of the precharge signal φ₄ is switched from the transistors N₃₁ and N₃₂ to the transistors N₃₃ and N₃₄. By selecting the sizes of the transistors N₃₃ and N₃₄ smaller (for instance by a factor of 1/5) than the transistor sizes of the transistors N₃₁ and N₃₂, the precharge control signal φ₄ presents a moderate slope indicated by m₃₂ that is determined by the ON resistances of the transistors N₃₃ and N₃₄. Consequently, the rise of the conductance of the transistor P₃₂ also becomes moderate or suppressed. Therefore, the charging current I_(cc3) becomes nearly constant. Subsequently, the node NO₃₅ becomes "1"-level via the delay circuit 32 after a delay time of the delay circuit 32, and the node NO₃₆ becomes "0"-level via the inverter circuit 33. Then, the node NO₃₂ again becomes "1"-level via the NAND circuit 34, and consequently, the transistor N₃₂ is turned ON. (The moment when the transistor N₃₂ is turned ON is indicated by t₃₂.) Thereby, the rate of change of the precharge control signal φ₄ again takes a steep slope indicated by m₃₃ towards OV. Therefore, the conductance of the transistor P₃₂ again starts to quickly rise, but at the moment t₃₂ in FIG. 4, the potential at the node NO₃₇ to be charged has been already come close to the V_(cc) potential (for instance 80% or higher of the V_(cc) potential), and hence the value of the charging current I_(cc3) begins to reduce. Subsequently, when the second input signal φ₂ becomes "0"-level, the transistor P₃₁ is turned ON, the transistors N₃₁ and N₃₃ are turned OFF, hence the precharge control signal φ₄ takes "1" level, and the transistor P₃₂ is turned OFF. Next, the first control signal φ₁ takes "1" level to bring the node NO₃₇ to be charged to "0"-level.

One example of application of this invention will now be shown. That case is where the above-described precharge control signal φ₄ is used as a precharge control signal for a digit line in a memory circuit. The explanation will be made with reference to FIGS. 5 and 6.

In FIG. 5, a memory cell MC is arrayed at an intersection of a word line WL and a pair of digit lines DL and DL. In this structure, the pair of digit lines DL and DL are precharged by P-channel transistors P₅₁,P₅₂ and P₅₃ in response to "0"-level of the precharge control signal φ₄.

The operation of the circuit in FIG. 5 will now be explained with reference to a timing chart in FIG. 6. The threshold voltage of the transistors P₅₁ to P₅₃ is represented by V_(TP). During read or write operations the precharge control signal φ₄ is at a "1"-level, and hence the transistors P₅₁ to P₅₃ are in an OFF state. In addition, at a time point when the above-mentioned operation has completed, one (DL) of the digit lines is at "0" level. Subsequently when it has transferred to a precharge state, the precharge control signal φ₄ begins to fall in potential, and if the potential becomes a potential equal to or lower than V_(cc) -|V_(TP) |, then the transistors P₅₁ to P₅₃ are turned ON, and thereby the digit line DL begins to be charged. As the potential of the precharge control signal φ₄ falls, the conductances of the transistors P₅₁ to P₅₃ are increased, and hence the value of the charging current I_(cc5) for the digit line DL becomes large gradually. Subsequently, when the slope of the precharge control signal φ₄ becomes moderate, the rise of the conductances of the transistors P₅₁ to P₅₃ becomes moderate. Consequently, the charging current I_(cc5) becomes nearly constant. After the potential of the DL line has risen to about 80 to 90% of the V_(cc) potential, the slope of the precharge control signal φ₄ again becomes steep, and the conductances of the transistors P₅₁ to P₅₃ begin to rise and the digit line rises towards the V_(cc) potential, but the charging current is reduced.

In general, in a memory circuit, the large peak current appearing as a charging current for digit lines, is a problem and one of the important points which must be considered when designing a memory circuit and also when designing a system employing the memory circuit. More particularly, although the capacitance of a digit line in a memory circuit is small (for instance 1 pF), as the memory increases in size, the number of digit lines is increased, and the total capacitance due to the digit lines is large. For instance, in a memory circuit of 16K bits, the number of the digit lines is sometimes 128, and hence in the prior art, the peak current value produced upon charging these digit lines amounted to as much as 150 mA. In a high-speed, large-capacity memory circuit, a large number of digit lines must be charged within a short period, and accordingly, as explained above with respect to the illustrated example, it is obvious that the present invention can bring about a large advantage.

Now, a second preferred embodiment of the present invention will be explained with reference to FIG. 7.

This embodiment shows an detailed example of the embodiment of FIG. 3. As will be apparent from the figure, in this preferred embodiment the logic blocks 31, 32, 33 and 34 in the above-described first preferred embodiment are replaced by corresponding circuits having transistors of CMOS construction. Accordingly, the operation is similar to the case of the above-described first preferred embodiment. Hence, only the correspondence between FIGS. 3 and 7 will be explained here. Logic circuits 31 through 34 are comprised of P-channel transistors P₇₂ to P₇₆ and N-channel transistors N₇₅ to N₇₉. The transistors P₇₂, N₇₅, P₇₃ and N₇₆ and the additive capacitance C₇₁ form a CMOS delay circuit 31 which corresponds to the delay circuit indicated with the same reference number in FIG. 3. The transistors P₇₆ and N₇₉ and the additive capacitance C₇₂ form a CMOS delay circuit and an inverter circuit in combination which corresponds to the delay circuit 32 and the inverter circuit 33 in FIG. 3. The transistors P₇₄, P₇₅, N₇₇ and N₇₈ form a CMOS NAND circuit 34 which corresponds to the NAND circuit 34 with the same reference number in FIG. 3.

Now a third preferred embodiment of the present invention will be explained with reference to FIGS. 8 and 9.

In FIG. 8, a precharge control signal generator portion 80 receives a second control signal φ₅ and generates a precharge control signals φ₆ to be applied to a precharge transistor. A precharge portion 85 includes an N-channel transistor N₈₂ coupled between the power supply terminal (V_(cc)) and a node NO₈₇ to be precharged and an N-channel transistor N₈₃ coupled between the node NO₈₇ and the ground potential. The transistor N₈₂ charges the node NO₈₇ in response to a "1" level of the precharge control signal φ₆ while the transistor N₈₃ discharges the node NO₈₇ in response to a "1"-level of the first control signal φ₁.

The precharge control signal generator portion 80 includes a series circuit of P-channel transistors P₈₁ and P₈₂ and a series circuit of P-channel transistors P₈₃ and P₈₄ both connected in series between the power supply terminal (V_(cc)) and an output node NO₈₀, and an N-channel transistor N₈₁ coupled between the node NO₈₀ and the ground potential. Delay circuits 81 and 82, an inverter 83 and a NOR gate 84 are provided for controlling the transistors P₈₂ and P₈₄.

Next will be described the operation of the preferred embodiment in FIG. 8 with reference to a timing chart shown in FIG. 9. When the first control signal φ₁ is at "1"-level, the transistor N₈₃ is turned ON, and so, the node NO₈₇ to be charged is brought to "0"-level. Subsequently, the first control signal φ₁ becomes "0"-level. Hence, the transistor N₈₃ is turned OFF. Then, when the second control signal φ₅ changes from "1" level to "0"-level, the transistor N₈₁ is turned OFF, while the transistors P₈₁ and P₈₃ are turned ON, so that the precharge signal φ₆ rises in potential at a slope m₈₁ that is determined by ON resistances of the respective transistors P₈₁ and P₈₂ starting from "0"-level. In response thereto, the transistor N₈₂ is turned ON to start charging of the node NO₈₇ to be charged. Since the conductance of the transistor N₈₂ becomes large, the charging current I_(cc8) for the node NO₈₇ to be charged would be increased gradually. On the other hand, when the second control signal φ₅ becomes "0"-level, the node NO₈₄ is brought to "0"-level through the delay circuit 81, the transistor P₈₄ is turned ON, the node NO₈₂ is brought to "1"-level via the NOR circuit 84, and the transistor P₈₂ is turned OFF. (The moment when the transistor P₈₂ is turned OFF is indicated by t₈₁.) In other words, at the moment t₈₁, the path of the current for potential rise of the precharge signal φ₆ is switched from the transistors P₈₁ and P₈₂ to the transistors P₈₃ and P₈₄. By selecting the transistor sizes of the transistors P₈₃ and P₈₄ smaller (for instance, by a factor of 1/5) than the transistor sizes of the transistors P₈₁ and P₈₂, the precharge signal φ₆ can take a moderate slope as indicated by m₈₂ which is determined by the ON resistances of the respective transistors P₈₃ and P₈₄. Consequently, the rise of the conductance of the transistor N₈₂ also becomes moderate, and so the charging current I_(cc8) becomes nearly constant. Subsequently, the node NO₈₅ is brought to the "0"-level via the delay circuit 82, the node NO₈₆ is brought to "1"-level via the inverter circuit 83, the node NO₈₂ is again brought to "0"-level via the NOR circuit 84 and the transistor P₈₂ is turned ON. (The moment when the transistor P₈₂ is turned on is indicated by t₈₂.) As a result, the precharge control signal φ₆ again takes a steep slope as indicated by m₈₃ and increases towards the V_(cc) potential.

Consequently, the conductance of the transistor N₈₂ again makes an abrupt rise. However, at the moment t₈₂ in FIG. 9, the potential of the node NO₈₇ to be charged is close to its final potential (for instance, assuming that the threshold voltage of the transistor N₈₂ is 0.8 V, a potential equal to or higher than 80% of V_(cc) -0.8 V), and thus the potential at the node NO₈₇ to be charged will approach its final potential while the value of the charging current I_(cc8) is being reduced. Subsequently, if the second control signal φ₅ becomes "1"-level, then the transistors P₈₁ and P₈₃ are turned OFF, the transistor N₈₁ is turned ON and the precharge control signal φ₆ becomes "0"-level. After a predetermined period, the first control signal φ₁ becomes "1"-level, so that the node NO₈₇ to be charged becomes "0"-level.

Three preferred embodiments and one example of an application of the invention have been described above. The advantages realized through the use of the present invention will now be explained.

In FIG. 10 are illustrated waveforms of the precharge signals and the charging current in the prior art and according to the present invention. In this figure, a waveform φ₃ is the precharge signal in the prior art shown in FIG. 1 and a waveform φ₄ shows a precharge signal according to the present invention. A waveform I_(cc1) shows a charging current for a node to be charged in the prior art, while a waveform I_(cc3) shows a charging current for a node to be charged according to the present invention. If potential fall arises just as the precharge control signal φ₃ in the prior art, then the conductance of the transistor P₁₂ shown in FIG. 1 would increase abruptly, and hence a large peak value appears in the charging current I_(cc1) for the node NO₁₁ to be charged. However, as is the case with the precharge signal φ₄ according to the present invention, if the slope of the potential fall is temporarily moderated at an intermediate potential, then the rise of the conductance of the transistor P₃₂ shown in FIG. 3 becomes moderate. Hence, the waveform of the charging current I_(cc3) for the node NO₃₇ to be charged takes the shape of a trapezoid, as compared to the wave form of an accute-angled isosceles triangle of the charging current I_(cc1). The peak value of the current can be reduced (for example up to 60% of the prior art) without extending the charging time and without changing the area of the figure formed of the waveform curve of the charging current and a portion of the time axis in FIG. 10, which area indicates the total quantity of the electric charges supplied to the node to be charged by the charging current. In general, with a printed circuit or the like on which individual parts are mounted, if the peak current upon operation of the parts is large, it would become a source of harmful noises, and therefore, reduction of the peak current is very effective to reduce noise.

It is to be noted that the present invention should not be limited to the above-described preferred embodiments, but obviously various other embodiments could be made within the scope which fulfils the essence of the present invention. 

I claim:
 1. A digital circuit comprising a first terminal to which a first potential is applied, a circuit node, a first transistor coupled between said first terminal and said circuit node, first means for receiving a precharge command signal, second means responsive to said precharge command signal for operatively generating a precharge control signal which changes from a first level towards a second level with a first time rate of change when a potential of said precharge control signal is near said first level and near said second level and with a second time rate of change smaller than said first time rate when the potential of said precharge control signal is near an intermediate level of said first and second levels, said first level making said first transistor non-conducting, said second level making said first transistor conducting, third means for directly supplying a control electrode of said first transistor with said precharge control signal without changing the waveform of said precharge control signal thereby to charge said circuit node with said first potential, and fourth means for selectively discharging said circuit node in response to an input signal only after said circuit node is charged by said third means, wherein a logic output is generated at said circuit node.
 2. The circuit according to claim 1, wherein said fourth means includes a second transistor coupled between said circuit node and a second terminal to which a second potential is applied.
 3. The circuit according to claim 1, in which said second means includes an output node, a first node to which a potential of said first level is applied, a second node to which a potential of said second level is applied, a second transistor coupled between said first node and said output node, third and fourth transistors coupled between said output node and said second node in parallel, the conductance of said third transistor being larger than that of said fourth transistor, means for rendering said second transistor conductive in response to said precharge command signal attaining a first value, means for rendering said third transistor conductive when said precharge command signal has attained a second value different from said first value and the voltage potential at said output node is not near said intermediate level, and means for causing said fourth transistor to conduct when the voltage potential at said output node is near said intermediate level.
 4. A precharge control circuit for generating a precharge control signal to be applied to a control electrode of a precharge transistor for precharging a data node with a precharge potential comprising:first and second transistor means for producing said precharge control signal in response to a precharge command signal having first and second logic levels the current paths of said first and second transistor means being provided between an output terminal and a predetermined potential in parallel, the conductance of said first transistor means being set substantially different from that of said second transistor means said predetermined potential rendering said precharge transistor conductive, circuit means for causing said first transistor means to be conductive and said second transistor means non-conductive when said precharge command signal changes from said first logic level to said second logic level and for causing said second transistor means to be conductive and said first transistor means non-conductive a predetermined time interval after the precharge command signal has started to change from said first to said second logic level, and for causing said first transistor means to again become conductive and said second transistor means non-conductive when said precharge command signal returns to its first logic level from its second logic level.
 5. The circuit of claim 4, wherein;said first transistor means is comprised of first and second field effect transistors, the gate of said first transistor being connected to receive said precharge command signal, the gate of said second field effect transistor being connected to a logic circuit for selectively causing said second transistor to conduct and, said second transistor means is comprised of third and fourth field effect transistors, the gate of said third transistor being connected to receive said precharge command signal, the gate of said fourth field effect transistor being connected to said logic circuit for selectively causing said fourth field effect transistor to conduct.
 6. The circuit of claim 5, wherein said logic circuit comprises first delay means connected to receive said precharge command signal, the output from said first delay means being connected to a second delay means, the gate of said fourth field effect transistor and a logic gate, the output of said logic gate being connected to said second field effect transistor, and inverter means for inverting the output from said second delay means and applying said inverted output as a second input to said logic gate.
 7. A digital circuit comprising: a first transistor with its drain-source coupled between a first voltage terminal and a data node, a second transistor with its drain-source coupled between said data node and a second voltage terminal, means responsive to a logic signal for selectively rendering said second transistor conductive, means for receiving a first signal, control means responsive to said first signal for generating a second signal having first and second levels at its output terminal, said second signal changing from said first level to said second level through consecutive first to third periods of time in response to said first signal, and means for applying said second signal to a gate of said first transistor, said second transistor assuming a conductive state and a non-conductive state in response to said second and first levels, respectively, said control means including third and fourth transistors, first connecting means for connecting a drain-source path of said third transistor between said output terminal and a predetermined potential corresponding to said second logic level, and second connecting means for connecting a drain-source path of said fourth transistor between said output terminal and said predetermined potential, the conductance of said third transistor being set substantially different from that of said fourth transistor, circuit means for causing said third transistor to be conductive and said fourth transistor to be non-conductive during said first and third periods of time and for causing said third transistor to be non-conductive and said fourth transistor to be conductive during said second period of time.
 8. The circuit acording to claim 7, wherein said first connecting means includes a fifth transistor whose drain-source path is connected in series with respect to said third transistor, the gate of said fifth transistor being connected to receive said first signal, and said second connecting means includes a sixth transistor whose drain-source path is connected in series with respect to said fourth transistor, the gate of said sixth transistor being connected to receive said first signal.
 9. The circuit according to claim 8, wherein said control means includes first delay means connected to receive said first signal, the output from said first delay means being connected to a second delay means, the gate of said fourth transistor and a logic gate, the output of said logic gate being connected to said third transistor, and inverter means for inverting the output from said second delay means and applying said inverted output as a second input to said logic gate.
 10. A digital circuit comprising: first means receiving a first signal which changes from one of two binary logic levels to the other of the binary logic levels with a first waveform of a single slope, second means responsive to a change of said first signal from the one of two binary logic levels towards the other of the binary logic levels for generating a second signal which changes from a first level to a second level with a second waveform, said second waveform assuming a first slope when said second signal is near an intermediate level of said first and second binary levels and with a second slope when said second signal is not near said intermediate level, the degree of said second slope being larger than that of said first slope, and transistor means connected to receive said second signal for generating a controlled amount of electric current in response to said second signal. 